Oak trees, crape myrtles, and camellias can get some "stuff" growing on their bark that looks unattractive. There are some products out there that have some degree of effectiveness for cleaning this stuff off of trees. A natural way is available if it doesn't bug you. Psocids, also known as tree cattle or bark lice, will clean the trunks of trees in the spring and fall. Typically, they will produce a lot of webbing and stay under it while eating the lichen and algae off of your tree.